The present invention relates to the field of horizontal oscillators for CRT devices and, more particularly, to an oscillator circuit which will prevent overload damage during loss of sync signals, and will allow a wide range of centering control.
In most raster scanning CRT devices, such as data display monitors, the oscillator which drives the horizontal output signal is designed to free-run at a rate slightly lower than the synchronized rate. Such monitors pose a particular problem since a loss of sync signal will cause the horizontal oscillator to revert to its much lower free running frequency and the resulting deflection current and high voltage may be high enough to overheat and cause permanent damage to the components in the horizontal output system. In the usual TV circuit, only one sync rate is used, and the oscillator is only required to pull-in for approximately 800 Hz. In monitors requiring a wider pull-in to accommodate various sync rates, a change of component values has been necessary.
A duty cycle of greater than 50% is needed in order to achieve proper scan for the monitor. It is also desirable to be able to adjust the delay of the horizontal output signals for centering the display within the raster and to accommodate other delays in the system. This combination of signal characteristics has been difficult to achieve with prior circuitry, and particularly at low cost.
It is an object, therefore, of the present invention to provide a horizontal oscillator which is operable over a range of frequencies and which will free-run at a frequency higher than the desired range of frequencies.
It is another object to provide such an oscillator with an adjustable delay in its sync signal supply.
It is still another object to provide such an oscillator with a duty cycle greater than 50%.
It is yet another object to provide such an oscillator at low cost.
These objects and others which will become apparent are achieved in an oscillator circuit having an adjustable delay circuit which can provide, approximately, a one-line range of delays in the horizontal sync pulse signal. The video signals can thus be centered in the raster regardless of the sync signal frequency, as well as compensating for any other delays in the horizontal circuitry of the CRT device. The oscillator is operable over the possible range of sync frequencies and will free-run at a frequency above that range.